The present invention relates to an aqueous suspension of collagen which is formed by a cross-linking reaction using a cross-linking agent, such as a polyepoxy compound. The cross-linked collagen is intracorporeally injectable and useful as repair material for soft tissue depression deficiency.
Collagen is a protein richly distributed in skin, cornea, vessels, sinew, bones and the like of animals. It is a sticky molecule with a molecular weight of about 0.3 million and has a helical structure consisting of three polypeptide chains with a molecular length of approximately 300 nm and a diameter of approximately 1.5 nm.
A bundle of five collagen molecules is called a microfibril and forms a structure separated by only 67 nm from the adjacent molecule. A microfibril has a thickness of about 4 nm and is the fundamental structural unit of collagen fiber. Many microfibrils comprise a fibril, and a bundle of fibrils comprises a collagen fiber. Sinew or the like is tissue in which the collagen fibers are arranged in an orderly fashion while skin is a tissue made up of entangled collagen fibers.
Fibrous collagen exhibits cross-linking between the molecule, and this cross-linking generally exists in the telopeptide showing no triple helix present at the terminal point of the collagen molecule. Atelocollagen can be obtained by digesting the telopeptide moiety with an enzyme such as pepsin. Since the main antigen determining group of collagen exists in the telopeptide moiety, the atelocollagen in which this telopeptide moiety has been digested hardly shows antigenicity and is therefore useful as medical material.
This atelocollagen is useful for repairing tissue deficient regions of soft tissues like skin by injecting an aqueous suspension of the atelocollagen into the depressed deficient wound, thereby eliminating the need for a surgical operation. The higher the collagen concentration of an aqueous collagen suspension during injection, the more effective the repairing effect is, due to the attendant skin swelling effect. It is preferable from the wound curing and cosmetic point of view to inject a small amount of such an aqueous suspension of atelocollagen at several sites rather than to inject a large amount thereof at one site. Therefore, it is necessary to use a thin needle, such as 27 G or 30 G. As such, a low viscosity is needed for injecting an aqueous suspension of atelocollagen. Thus, the intracorporeally injectable aqueous suspension of atelocollagen should have a high concentration and a low viscosity.
Three types of intracorporeally injectable collagen are proposed. For simplicity, they are referred to below as Collagen A, Collagen B and Collagen C. Collagen A is an aqueous neutral solution of collagen (JP Patent Publication No. Sho 62-37020). Collagen B is an aqueous suspension of collagen fibers, and Collagen C is an aqueous suspension of atelocollagen cross-linked with glutaraldehyde (JP Patent Publication No. Hei 1-36840).
The aqueous neutral solution type Collagen A is viscous at low temperatures. Implantation of this type of collagen into a living body induces the formation of collagen fibers by body temperature, whereby the same collagen structure as living collagen is constituted. Decomposition and absorption of this type of collagen within living bodies occurs slowly and produces a high skin swelling effect. However, it is difficult to inject a highly concentrated solution of Collagen A because of its high viscosity. Injection of a large amount of a highly concentrated solution at one site would form a collagen structure which would tend to prevent the live self-organizing cells from penetrating therein.
An aqueous suspension of collagen fibers (Collagen B) is prepared by subjecting collagen molecules to conditions similar to those of living bodies and dispersing the resulting fibers in water. Since it is difficult to inject the material using a fine needle as is, the fibrous structure must be converted into a very fine structure, in order to allow easy injection. Once the collagen is implanted into a living body, it diffuses below the skin soon after injection, and decomposition and absorption take place quickly. Thus, the skin swelling effect is so low that injection would have to be repeatedly performed to retain the desired result. Certain suspensions of highly concentrated collagen have been developed for overcoming this defect, but the diffusion, decomposition and absorption of such suspensions in a living body occur so rapidly that marked improvement of the skin via the swelling effect has not been observed.
In the case of Collagen B materials, which are not cross-linked, 2 to 3% of persons have been found to be reactive, even though the telopeptide, the main antigenicity-determining group, has been removed. Repeated administration of Collagen B would also likely produce new reactions in 2 to 3% of the previous non-reactive persons. This, of course is a major problem.
Atelocollagen cross-linked with glutaraldehyde (Collagen C) was developed to try to improve the aforesaid problem. Collagen C is an aqueous suspension of a new material prepared by cross-linking atelocollagen with glutaraldehyde, which raises resistance to intracorporeal decomposition and absorption while further lowering antigenicity. However, when atelocollagen is cross-linked with glutaraldehyde, it becomes water-repellent and its fluidity in aqueous suspensions is inferior to that of atelocollagen which is not cross-linked. In particular, the fluidity of a high concentration of Collagen C makes it difficult to inject. As for intracorporeal change of the implanted material with the passage of time, calcification and the like are often observed, together with low identification into self-tissue. This tendency is pronounced at high concentrations. Therefore, it is not appropriate to inject a high concentration of such cross-linked atelocollagen because the skin swelling effect in such a case would only be improved somewhat when compared with that of atelocollagen not cross-linked. The use of glutaraldehyde as a cross-linking agent also poses a problem because of its cytotoxicity.